Radiographic apparatus and radiation detection signal processing method

ABSTRACT

A radiographic apparatus according to this invention stores, before an imaging event, offset images and gain correcting images corresponding to a plurality of storage times, and acquires a lag image and a radiographic image based on these stored images. Then, lag correction is carried out to remove lags, using the lag image, from the radiographic image. In this way, from the radiographic image taking into consideration the offset images and gain correcting images corresponding to the storage times, lags are removed using the lag image which similarly fakes into consideration the offset images and gain correcting images corresponding to the storage times. Lag-behind parts, including offset and gain components, are removed from radiation detection signals in a simple way.

TECHNICAL FIELD

This invention relates to a radiographic apparatus and a radiationdetection signal processing method for obtaining radiographic imagesbased on radiation detection signals resulting from radiation emitted toand transmitted through an object under examination. More particularly,the invention relates to a technique for eliminating lag-behind partsfrom the radiation detection signals.

BACKGROUND ART

An example of radiographic apparatus is an imaging apparatus thatobtains X-ray images by detecting X rays. This apparatus used an imageintensifier as an X-ray detecting device in the past. In recent years, aflat panel X-ray detector (hereinafter called simply “FPD”) has come tobe used instead.

The FPD has a sensitive film laminated on a substrate, detects radiationincident on the sensitive film, converts the detected radiation intoelectric charges, and stores the electric charges in capacitors arrangedin a two-dimensional array. The electric charges are read by turning onswitching elements, and are transmitted as radiation detection signalsto an image processor. The image processor obtains an image havingpixels based on the radiation detection signals.

The FPD is lightweight and free from complicated detecting distortionscompared with the image intensifier used heretofore. Thus, the, FPD hasadvantages in terms of apparatus construction and image processing.

However, when the FPD is used, the X-ray detection signals includelag-behind parts. A lag-behind part results in an afterimage from X-rayirradiation in a preceding imaging event appearing as an artifact on anext X-ray image. Particularly, in a fluoroscopy that performs X-rayirradiation continually at short time intervals (e.g. 1/30 second), timelags of the lag-behind parts have influences serious enough to hinderdiagnosis.

Artifacts due to lag-behind parts are reduced by reducing long timeconstant components of the lag-behind parts by using backlight (seePatent Document 1, for example), or by regarding the lag-behind parts asa total of exponential functions having a plurality of time constants,and performing a lag correction by recursive computation using theseexponential functions (see Patent Document 2, for example).

Where backlight is used as disclosed in the Patent Document 1 notedabove, the construction becomes complicated by a construction requiredfor backlight. Particularly where backlight is used in an FPD having alightweight construction, the construction must become heavy andcomplicated again. In the case of Patent Document 2, the lag correctionmust be carried out by performing recursive computations the number oftimes X-ray detection signals are sampled. This renders the lagcorrection complicated and cumbersome.

In order to remove lag-behind parts included in X-ray detection, signalssimply from the X ray detection signals, it is conceivable in performinga lag correction, to acquire a plurality of X-ray detection signals intime of non-irradiation before irradiation of X rays in an imagingevent, acquire a lag image based on the X-ray detection signals, andusing this image to remove the lags from a product X-ray image.

In addition to lag correction, correction processes include offsetcorrection, gain correction, and defect correction, for example. Toperform offset correction, for example, an offset image is obtainedbeforehand in time of non-irradiation. The above offset image issubtracted from an original image based on X-ray detection signals. Anoffset image is different for each mode such as storage time,amplification, factor (gain) of an amplifier or pixel binning (additionof a plurality of pixels). An offset image according to a mode isobtained to perform offset correction, (see Patent Document 3, forexample). Pixel binning includes a 1×1 mode which outputs pixels in 1 to1, a 2×2 mode which outputs four pixels of 2×2 in rows and columns toone pixel, and a 4×4 mode which outputs 16 pixels of 4×4 in rows andcolumns to one pixel.

[Patent Document 1]

Unexamined Patent Publication No. H9-9153 (pages 3-8, FIG. 1)

[Patent Document 2]

Unexamined Patent Publication No. 2004-242741 (pages 4-11, FIGS. 1 and3-6)

[Patent Document 3]

Unexamined Patent Publication No. 2003-190126 (pages 3-6, FIG. 1)

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

However, there are the following problems with the technique of removinglags from a product X-ray image using the above acquired lag image. TheX-ray detection signals acquired in time of non-irradiation includeoffset components due to dark current. With this technique, therefore,both offset, and lag components can be corrected simultaneously byremoving lags, using a lag image including the offset components, froman uncorrected X-ray image immediately after image pick-up and nothaving undergone offset correction or gain correction.

In practice, however, the offset components (i.e. offset values) have adifferent property according to a storage time for accumulating signalinformation (electric charges). It is therefore necessary to subtractthe offset values corresponding to the storage time used at an imagepick-up time. In practice, the storage time used at an image pick-uptime is dependent on a pulse width of X rays variable in time with athickness and the like of an object under examination. It is impossibleto determine the storage time at the time of image pick-up beforehand.

On the other hand, when collecting a plurality of X-ray detectionsignals in time of non-irradiation in order to acquire a lag image, thesignals should be collected within a minimum storage time. Otherwise,the lag collection itself will consume a long time, which isundesirable. Thus, the collection for acquisition of a lag image and anactual image pick-up have to be performed for different storage times(different modes). This requires an improvement for enabling lagcorrection even for the different storage times. For gain correctionalso, it is necessary to use an image for gain correction according todifferent storage times.

This invention has been made having regard to the state of the art notedabove, and its object is to provide a radiographic apparatus and aradiation detection signal processing method for eliminating lag-behindparts, including offset or gain components, from radiation detectionsignals in a simple way.

Means for Solving the Problem

To fulfill the above object, this invention employs the followingconstruction.

A radiographic apparatus according to this invention is a radiographicapparatus for obtaining radiographic images based on radiation detectionsignals comprising a radiation emitting device for emitting radiationtoward an object under examination; a radiation detecting device fordetecting radiation transmitted through the object; an offset imagestorage device for storing offset images corresponding to a plurality ofstorage times for accumulating information on the signals, the offsetimages being used to perform offset correction for removing offsetvalues superimposed on the signals; a non-irradiation signal, acquiringdevice for acquiring a plurality of radiation detection signals detectedfrom the radiation detecting device in time of non-irradiation beforeirradiation of the radiation in an imaging event; a lag image acquiringdevice for acquiring a lag image based on the radiation detectionsignals acquired by the non-irradiation signal acquiring device, and theoffset images stored in said offset image storage device andcorresponding to the storage times for the non-irradiation signalacquiring device; an irradiation signal acquiring device for acquiringthe radiation detection signals detected, from the radiation detectingdevice in time of irradiation of the radiation in the imaging event; aradiographic image acquiring device for acquiring a radiographic imageserving an intended purpose based on the radiation detection signalsacquired by the irradiation signal acquiring device, and the offsetimages stored in said offset image storage device and corresponding tothe storage times for the irradiation signal acquiring device; and a lagcorrecting device for removing lags, using the lag image acquired bysaid lag image acquiring device, from the radiographic image acquired bythe radiographic image acquiring device, thereby performing a lagcorrection of lag-behind parts by removing the lag-behind parts from theradiation detection signals.

According to the radiographic apparatus of this invention, offset imagesare used to carry out offset correction for removing offset valuessuperimposed on signals. The offset images corresponding to a pluralityof storage times for accumulating signal information are stored in theoffset image storage device. The non-irradiation signal acquiring deviceacquires a plurality of radiation detection signals detected from theradiation detecting device in time of non-irradiation before irradiationof the radiation in an imaging event. Based on these radiation detectionsignals acquired by the non-irradiation signal acquiring device, theoffset images stored in the above offset image storage device andcorresponding to the storage times for the non-irradiation signalacquiring device, the lag image acquiring device acquires a lag image.On the other hand, the irradiation signal acquiring device acquiresradiation detection, signals detected from the radiation detectingdevice in time of irradiation of the radiation in the imaging event.Based on the radiation detection signals acquired by the irradiationsignal acquiring device, and the offset images stored in the aboveoffset image storage device and corresponding to the storage times forthe irradiation signal acquiring device, the radiographic imageacquiring device acquires a radiographic image serving the intendedpurpose. The lag correcting device removes lags, using the lag imageacquired by the lag image acquiring device, from the radiographic imageacquired by the radiographic image acquiring device, thereby performinga lag correction of lag-behind parts by removing the lag-behind partsfrom the radiation detection, signals. Thus, there is no need to carryout lag correction by performing recursive computations the number oflimes radiation detection signals are sampled, as described in PatentDocument 2 noted hereinbefore. Further, the lag image forming the basisfor the above lag correction, and the radiation image which is thetarget of the lag correction, take into consideration the offset imagesand lag correcting images corresponding to the respective storage times,it becomes possible to perform appropriately also offset correctionaccording to the storage times by lag correction. A lag-behind part maytherefore be eliminated from a radiation detection signal in a simpleway. Further, there is no need to use backlight as used in PatentDocument 1 noted hereinbefore. This avoids complication of the apparatusconstruction.

A different radiographic apparatus according to this invention is aradiographic apparatus for obtaining radiographic images based onradiation detection signals, comprising a radiation emitting device foremitting radiation toward an object under examination; a radiationdetecting device for detecting radiation transmitted through the object;a gain correcting image storage device for storing gain correctingimages corresponding to a plurality of storage times for accumulatinginformation on the signals, the gain correcting images being used toperform gain correction for equalizing signal levels of pixels to beoutputted; a non-irradiation signal acquiring device for acquiring aplurality of radiation detection signals detected from the radiationdetecting device in time of non-irradiation before irradiation of theradiation in an imaging event; a lag image acquiring device foracquiring a lag image based on the radiation detection signals acquiredby the non-irradiation signal acquiring device, and the gain correctingimages stored in said gain correcting image storage device andcorresponding to the storage times for the non-irradiation signalacquiring device; an irradiation signal acquiring device for acquiringthe radiation detection signals detected from the radiation detectingdevice in time of irradiation of the radiation in the imaging event; aradiographic image acquiring device for acquiring a radiographic imageserving an intended purpose based on the radiation detection signalsacquired by the irradiation signal acquiring device, and the gaincorrecting images stored in said gain correcting image storage deviceand corresponding to the storage times for the irradiation signalacquiring device; and a lag correcting device for removing lags, usingthe lag image acquired by said lag image acquiring device, from theradiographic image acquired by the radiographic image acquiring device,thereby performing a lag correction of lag-behind parts by removing thelag-behind parts from the radiation detection signals.

According to the different radiographic apparatus of this invention,gain correcting images are used to carry out gain correction forequalizing signal levels of pixels to be outputted. The gain correctingimages corresponding to a plurality of storage times for accumulatingsignal information are stored in the gain correcting image storagedevice. The non-irradiation signal acquiring device acquires a pluralityof radiation detection signals detected from the radiation detectingdevice in time of non-irradiation before irradiation of the radiation inan imaging event. Based on these radiation detection signals acquired bythe non-irradiation signal acquiring device, the gain correcting imagesstored in the above gain correcting image storage device andcorresponding to the storage times for the non-irradiation signalacquiring device, the lag image acquiring device acquires a lag image.On the other hand, the irradiation signal acquiring device acquiresradiation detection signals detected from the radiation detecting devicein time of irradiation of the radiation in the imaging event. Based onthe radiation detection signals acquired by the irradiation signalacquiring device, and the gain correcting images stored in the abovegain correcting image storage device and corresponding to the storagetimes for the irradiation signal acquiring device, the radiographicimage acquiring device acquires a radiographic image serving theintended purpose. The lag correcting device removes lags, using the lagimage acquired by the lag image acquiring device, from the radiographicimage acquired by the radiographic image acquiring device, therebyperforming a lag correction of lag-behind parts by removing thelag-behind parts from the radiation detection signals. Thus, there is noneed to carry out lag correction by performing recursive computationsthe number of times radiation detection signals are sampled, asdescribed in Patent Document 2 noted hereinbefore. Further, the lagimage forming the basis for the above lag correction, and the radiationimage which is the target of the lag correction, take into considerationthe gain correcting images and lag correcting images corresponding tothe respective storage times. If becomes possible to performappropriately also gain correction according to the storage times by lagcorrection, A lag-behind part may therefore be eliminated from aradiation detection signal in a simple way. Further, there is no need touse backlight as used in. Patent Document 1 noted hereinbefore. Thisavoids complication of the apparatus construction.

A radiation detection signal processing method according to thisinvention is a radiation detection signal processing method forperforming a signal processing to obtain radiographic images based onradiation detection signals detected by irradiating an object underexamination, said signal processing comprising an offset image storingstep for storing, before an imaging event, offset images correspondingto a plurality of storage times for accumulating information on thesignals, the offset images being used to perform offset correction forremoving offset values superimposed on the signals; a non-irradiationsignal acquiring step for acquiring a plurality of radiation detectionsignals in time of non-irradiation before irradiation of the radiationin the imaging event; a lag image acquiring step for acquiring a lagimage based on the radiation detection signals acquired in thenon-irradiation signal acquiring step, and the offset images stored insaid offset image storage step and corresponding to the storage times inthe non-irradiation signal acquiring step; an irradiation signalacquiring step for acquiring the radiation detection signals in time ofirradiation of the radiation in the imaging event; a radiographic imageacquiring step for acquiring a radiographic image serving an intendedpurpose based on the radiation detection signals acquired in theirradiation signal acquiring step, and the offset images stored in saidoffset, image storage step and corresponding to the storage times in theirradiation signal acquiring step; and a lag correcting step forremoving lags, using the lag image acquired in said lag image acquiringstep, from the radiographic image acquired in the radiographic imageacquiring step, thereby performing a lag correction of lag-behind partsby removing the lag-behind parts from the radiation detection signals.

According to the radiation detection signal processing method of thisinvention, offset images are used to carry out offset correction forremoving offset values superimposed on signals. The offset imagescorresponding to a plurality of storage times for accumulating signalinformation are stored in the offset image storing step before animaging event. The non-irradiation signal acquiring step acquires aplurality of radiation detection signals in time of non-irradiationbefore irradiation of the radiation in the imaging event. Based on theseradiation detection signals acquired in the non-irradiation signalacquiring step, the offset images stored in the above offset imagestoring step and corresponding to the storage times for thenon-irradiation signal acquiring step, the lag image acquiring stepacquires a lag image. On the other hand, the irradiation signalacquiring step acquires radiation detection signals in time ofirradiation of the radiation in the imaging event. Based on theradiation detection signals acquired in the irradiation signal acquiringstep, and the offset images stored in the above offset image storingstep and corresponding to the storage times for the irradiation signalacquiring step, the radiographic image acquiring step acquires aradiographic image serving the intended purpose. The lag correcting stepremoves lags, using the lag image acquired in the lag image acquiringstep, from the radiographic image acquired in the radiographic imageacquiring step, thereby performing a lag correction of lag-behind partsby removing the lag-behind parts from the radiation detection signals.Thus, there is no need to carry out lag correction by performingrecursive computations the number of times radiation detection signalsare sampled, as described, in Patent Document 2 noted hereinbefore.Further, the lag image forming the basis for the above lag correction,and the radiation image which is the target of the lag correction, takeinto consideration the offset images and lag correcting imagescorresponding to the respective storage times. It becomes possible toperform appropriately also offset correction according to the storagetimes by lag correction. A lag-behind part may therefore be eliminatedfrom a radiation detection signal in a simple way.

A different radiation detection signal processing method according tothis invention is a radiation detection signal processing method forperforming a signal processing to obtain radiographic images based onradiation detection signals detected by irradiating an object underexamination, said signal processing comprising a gain correcting imagestoring step for storing, before an imaging event, gain correctingimages corresponding to a plurality of storage times for accumulatinginformation on the signals, the gain correcting images being used toperform gain correction for equalizing signal levels of pixels to beoutputted; a non-irradiation signal acquiring step for acquiring aplurality of radiation detection signals in time of non-irradiationbefore irradiation of the radiation in the imaging event; a lag imageacquiring step for acquiring a lag image based on the radiationdetection signals acquired in the non-irradiation signal acquiring step,and the gain correcting images stored in said gain correcting imagestorage step and corresponding to the storage times in thenon-irradiation signal acquiring step; an irradiation signal acquiringstep for acquiring the radiation detection signals in time ofirradiation of the radiation in the imaging event; a radiographic imageacquiring step for acquiring a radiographic image serving an intendedpurpose based on the radiation detection signals acquired in theirradiation signal acquiring step, and the gain correcting images storedin said gain correcting image storage step and corresponding to thestorage times in the irradiation signal acquiring step; and a lagcorrecting step for removing lags, using the lag image acquired in saidlag image acquiring step, from the radiographic image acquired in theradiographic image acquiring step, thereby performing a lag correctionof lag-behind parts by removing the lag-behind parts from the radiationdetection signals.

According to the different radiation detection signal processing methodof this invention, gain correcting images are used to carry out gain,correction for equalizing signal levels of pixels to be outputted. Thegain correcting images corresponding to a plurality of storage times foraccumulating signal information are stored in the gain correcting imagestoring step before an imaging event. The near irradiation signalacquiring step acquires a plurality of radiation detection, signals intime of non-irradiation, before irradiation of the radiation in theimaging event. Based on these radiation detection signals acquired, inthe non-irradiation signal acquiring step, the gain correcting imagesstored in the above gain correcting image storing step and correspondingto the storage times for the non-irradiation signal acquiring step, thelag image acquiring step acquires a lag image. On the other hand, theirradiation signal acquiring step acquires radiation detection signalsin time of irradiation of the radiation in the imaging event. Based onthe radiation detection signals acquired in the irradiation signalacquiring step, and the gain, correcting images stored in the above gaincorrecting image storing step and corresponding to the storage times forthe irradiation signal acquiring step, the radiographic image acquiringstep acquires a radiographic image serving the intended purpose. The lagcorrecting step removes lags, using the lag image acquired in the lagimage acquiring step, from the radiographic image acquired in theradiographic image acquiring step, thereby performing a lag correctionof lag-behind parts by removing the lag-behind parts from the radiationdetection signals. Thus, there is no need to carry out lag correction byperforming recursive computations the number of times radiationdetection signals are sampled, as described in Patent Document 2 notedhereinbefore. Further, the lag image forming the basis for the above lagcorrection, and the radiation image which is the target of the lagcorrection, take into consideration the gain correcting images and lagcorrecting images corresponding to the respective storage times. Itbecomes possible to perform appropriately also gain correction accordingto the storage times by lag correction. A lag-behind part may thereforebe eliminated from a radiation detection signal in a simple way.

EFFECTS OF THE INVENTION

With the radiographic apparatus and radiation detection, signalprocessing method, according to this invention, from a radiographicimage taking into consideration offset images and gain correcting imagescorresponding to storage times, lags are removed using a lag image whichsimilarly takes into consideration the offset images and gain correctingimages corresponding to the storage times. Thus, lag-behind parts,including offset and gain components, included in radiation detectionsignals are removed from the radiation detection signals in a simpleway.

BRIEF DESCRIPTION OF THE DRAWINGS

[FIG. 1]Block diagram of a fluoroscopic apparatus according to theinvention.

[FIG. 2]Equivalent circuit, seen in side view, of a flat panel X-raydetector used in the fluoroscopic apparatus.

[FIG. 3]Equivalent circuit, seen in plan view, of the flat panel X-raydetector.

[FIGS. 4](a) and (b) are explanatory views schematically showing offsetcorrection.

[FIG. 5](a) and (b) are explanatory views schematically showing gaincorrection.

[FIG. 6]Flow chart showing a series of signal processing by anon-irradiation signal acquiring unit, a lag image acquiring unit, anirradiation signal acquiring unit, an X-ray image acquiring unit and alag correcting unit in a first embodiment.

[FIG. 7]Time chart showing X-ray emissions and acquisition of X-raydetection signals.

[FIG. 8]Schematic view showing storage of offset images gain correctingimages corresponding to storage times.

[FIG. 9]Schematic view showing flows of data to and from an imageprocessor and a memory in the first and second embodiments.

[FIG. 10]Flow chart showing a series of signal processing by anon-irradiation signal acquiring unit, a lag image acquiring unit, anirradiation signal acquiring unit, an X-ray image acquiring unit and alag correcting unit in the second embodiment.

[FIG. 11]Schematic view showing flows of data to and from an imageprocessor and a memory in a third embodiment.

[FIG. 12]Flow chart showing a series of signal processing by anon-irradiation signal acquiring unit, a lag image acquiring unit, anirradiation signal acquiring unit, an X-ray image acquiring unit and alag correcting unit in the third embodiment.

[FIG. 13]Schematic view showing variations of random noise occurringwith the frequency of recursive computation when load ratios are changedin the third embodiment.

DESCRIPTION OF REFERENCES

2 . . . X-ray tube

3 . . . flat panel X-ray detector (FPD)

9 a . . . non-irradiation signal acquiring unit

9 b . . . lag image acquiring unit

9 c . . . irradiation signal acquiring unit

9 d . . . X-ray image acquiring unit

9 e . . . lag correcting unit

11 a . . . offset image memory unit

11 b . . . gain correcting image memory unit

X, X′, Y . . . X-ray image

L, L′. . . lag image

M . . . patient

BEST MODE FOR CARRYING OUT THE INVENTION

In a radiation detection signal processing method, offset images andgain correcting images corresponding to a plurality of storage times arestored in memory before an imaging event, and a lag image is acquiredand a radiographic image is acquired based on these stored images. Lagcorrection is carried out by removing lags, using the lag image, fromthe radiographic image. From the radiographic image taking intoconsideration the offset images and gain correcting images correspondingto the storage times, lags are removed using the lag image whichsimilarly takes into consideration the offset images and gain correctingimages corresponding to the storage times. Thus, the object ofeliminating lag-behind parts, including offset and gain components, fromradiation detection signals in a simple way has been fulfilled.

Embodiment 1

Embodiment 1 of this invention will be described hereinafter withreference to the drawings. FIG. 1 is a block diagram of a fluoroscopicapparatus in Embodiment 1. FIG. 2 is an equivalent circuit, seen in sideview, of a flat panel X-ray detector used in the fluoroscopic apparatus.FIG. 3 is an equivalent circuit, seen in plan view, of the flat panelX-ray detector. Embodiment 1, and also Embodiments 2 and 3 to follow,will be described, taking the fiat panel X-ray detector (hereinaftercalled “FPD” as appropriate) as an example of radiation detectiondevice, and the fluoroscopic apparatus as an example of radiographicapparatus.

As shown in FIG. 1, the fluoroscopic apparatus in Embodiment 1 includesa top board 1 for supporting a patient M, an X-ray tube 2 for emitting Xrays toward the patient M, and an FPD 3 for detecting X rays transmittedthrough the patient M. The X-ray tube 2 corresponds to the radiationemitting device in this invention. The FPD 3 corresponds to theradiation detecting device in this invention.

The fluoroscopic apparatus further includes a top hoard controller 4 forcontrolling vertical and horizontal movements of the top board 1, an FPDcontroller 5 for controlling scanning action of the FPD 3, an X-ray tubecontroller 7 having a high voltage generator 6 for generating a tubevoltage and tube current for the X-ray tube 2, an analog-to-digitalconverter 8 for fetching charge signals from the FPD 3 and digitizingthe charge signals into X-ray detection signals, an image processor 9for performs various processes based on the X-ray detection signalsoutputted from the analog-to-digital converter 8, a controller 10 forperforming an overall control of these components, a memory 11 forstoring processed images, an input unit 12 for the operator to input,various settings, and a monitor 13 for displaying the processed imagesand other information.

The top board controller 4 controls movements of the top board 1, suchas moving the top board 1 horizontally to place the patient M in animaging position, vertically moving and/or rotating the top board 1 toset the patient M to a desired position, horizontally moving the topboard 1 during an imaging operation, and horizontally moving the topboard 1 to withdraw the patient M from the imaging position after theimaging operation. The FPD controller 5 controls scanning action bymoving the FPD 3 horizontally or revolving the FPD 3 about the body axisof patient M. The high voltage generator 8 generates the tube voltageand tube current for the X-ray tube 2, to emit X rays. The X-ray tubecontroller 7 controls scanning action by moving the X-ray tube 2horizontally or revolving the X-ray tube 2 about the body axis ofpatient M, and controls setting of a coverage of a collimator (notshown) disposed adjacent the X-ray tube 2. In time of scanning action,the X-ray tube 2 and FPD 3 are moved while maintaining a mutuallyopposed relationship, so that the FPD 3 may detect X rays emitted fromthe X-ray tube 2.

The controller 10 has a central processing unit (CPU) and otherelements. The memory 11 has storage media, typically a ROM (Read-OnlyMemory) and RAM (Random Access Memory. The input, unit 12 has a pointingdevice, typically a mouse, keyboard, joy stick, trackball and/or touchpanel. The fluoroscopic apparatus creates images of the patient M, withthe FPD 3 detecting X rays transmitted through the patient M, and theimage processor 9 performing an image processing based on the X raysdetected.

The image processor 9 includes a non-irradiation signal acquiring unit 9a for acquiring a plurality of X-ray detection signals in time ofnon-irradiation before X-ray irradiation in an imaging event, a lagimage acquiring unit 9 b for acquiring a lag image based on offsetimages and gain correcting images described hereinafter, an irradiationsignal acquiring unit 9 c for acquiring X-ray detection signals in timeof X-ray irradiation in an imaging event, and an X-ray image acquiringunit 9 d for acquiring an X-ray image serving an intended purpose basedon the X-ray detection signals acquired by the irradiation signalacquiring unit 9 c and the offset images and gain correcting imagesdescribed hereinafter, and a lag correcting unit 9 e for removing lagsfrom the X-ray image acquired by the X-ray image acquiring unit 9 d byusing the lag image acquired by the lag image acquiring unit 9 b. Thelag correcting unit 9 e removes lags from the X-ray image by using thelag image to remove any lag-behind parts from the X-ray detectionsignals, thereby performing a lag correction of the lag-behind parts.The non-irradiation signal acquiring unit 9 a corresponds to thenon-irradiation signal acquiring device in this invention. The lag imageacquiring unit 9 b corresponds to the lag image acquiring device in thisinvention. The irradiation signal acquiring unit 9 c corresponds to theirradiation signal acquiring device in this invention. The X-ray imageacquiring unit 9 d corresponds to the radiation image acquiring devicein this invention. The lag correcting unit 9 e corresponds to the lagcorrecting device in this invention.

The memory 11 includes an offset image memory unit 11 a for storingoffset images corresponding to a plurality of storage times foraccumulating signal information, a gain correcting image memory unit 11b for storing gain correcting images likewise corresponding to aplurality of storage times, a non-irradiation signal memory unit lie forstoring X-ray detection signals acquired by the non-irradiation signalacquiring unit 9 a in time of non-irradiation, a lag image memory unit11 d for storing the lag image acquired by the lag image acquiring unit9 b, an irradiation signal memory unit lie for storing X-ray detectionsignals acquired by the irradiation signal acquiring unit 9 c in time ofirradiation, and an X-ray image memory unit 11 f for storing X-rayimages acquired by the X-ray image acquiring unit 9 d. In Embodiment 1,and also in Embodiment 2 described hereinafter, the lag image acquiringunit 9 b acquires a lag image based on the X-ray detection signals ofnon-irradiation times read from the non-irradiation signal memory unit11 c (see FIG. 9). In Embodiment 3 to follow, a lag image is obtained bya recursive weighted average (recursive process) as describedhereinafter (see FIG. 11). The offset image memory unit 11 a correspondsto the offset image storage device in this invention. The gaincorrecting image memory unit 11 b corresponds to the gain correctingimage storage device in this invention.

As shown in FIG. 2, the FPD 3 includes a glass substrate 31, and thinfilm transistors TFT formed on the glass substrate 31. As shown in FIGS.2 and 3, the thin film transistors TFT comprise numerous (e.g.1,024×1,024) switching elements 32 arranged in a two-dimensional matrixof rows and columns. The switching elements 32 are formed separate fromone another for respective carrier collecting electrodes 33. Thus, theFPD 3 is also a two-dimensional array radiation detector.

As shown in FIG. 2, an X-ray sensitive semiconductor 34 is laminated onthe carrier collecting electrodes 33. As shown in FIGS. 2 and 3, thecarrier collecting electrodes 33 are connected to the sources S of theswitching elements 32. A plurality of gate bus lines 36 extend from agate driver 35, and are connected to the gates G of the switchingelements 32. On the other hand, as shown in FIG. 3, a plurality of databus lines 39 are connected through amplifiers 38 to a multiplexer 37 forcollecting charge signals and outputting as one. As shown in FIGS. 2 and3, each data bus line 39 is connected to the drains D of the switchingelements 32.

With a bias voltage applied to a common electrode not shown, the gatesof the switching elements 32 are turned on by applying thereto (orreducing to 0V) the voltage of the gate bus lines 38. The carriercollecting electrodes 33 output charge signals (carriers) converted fromX rays incident on the detection surface through the X-ray sensitivesemiconductor 34, to the data bus lines 39 through, the sources S anddrains D of the switching elements 32. The charge signals areprovisionally stored in capacitors (not shown) until the switchingelements are turned on. The amplifiers 38 amplify the charge signalsread, out to the data bus lines 39, and the multiplexer 37 collects thecharge signals, and outputs them as one charge signal. Theanalog-to-digital converter 8 digitizes the outputted charge signal, andoutputs it as an X-ray detection signal.

Next, offset correction and gain correction will be described withreference to the explanatory views in FIGS. 4 and 5. FIGS. 4 and 5illustrate, by way of example, four pixels arranged in two rows and twocolumns. The offset, images noted above are used to perform offsetcorrection, for removing offset values superimposed on signals. The gaincorrecting images noted above are used to perform gain correction forequalizing signal levels of pixels to be outputted.

Offset components (i.e. offset values) are superimposed, on signallevels of the pixels (i.e. pixel values) based on the X-ray detectionsignals when, outputted from the image processor 9. Specifically, asshown in FIG. 4 (a), offset values O (={O₁₁, O₁₂, O₂₁, O₂₂}) due to darkcurrent are outputted in time of non-irradiation. As shown in FIG. 4(b), the offset values O in time of non-irradiation are superimposed onpixel values S (={S₁₁, S₁₂, S₂₁, S₂₂}) to output values (S+O)(={S₁₁+O₁₁, S₁₂+O₁₂, S₂₁+O₂₁, S₂₂+O₂₂}). Thus, offset values O in timeof non-irradiation, i.e., offset images, are determined and stored inthe offset image memory unit 11 a in advance of an imaging event.

On the other hand, while the signal levels of the pixels (i.e. pixelvalues) based on the X-ray detection signals are outputted according tothe value of the amplification factor (gain) of the amplifier 38 of FPD3 (see FIG. 3), pixel values S outputted are variable from pixel topixel due to individual specificities such as of the switching elements32 corresponding to the respective pixels. Specifically, assume that, asshown in FIG. 5 (a), varied pixel values S {S₁₁, S₁₂, S₂₁, S₂₂} areoutputted when the same dose of X rays is incident on each pixel. It isfurther assumed that, by adjusting each gain, as shown in FIG. 5 (b),pixel values S outputted are equalized to be {S_(O), S_(O), S_(O),S_(O)}. Thus, gain correcting images G are determined as {S₁₁/S_(O),S₁₂/S_(O), S₂₁/S_(O), S₂₂/S_(O)} and stored in the gain correcting imagememory unit 11 b in advance of an imaging event.

Next, a series of signal processing by the non-irradiation signalacquiring unit 9 a, lag image acquiring unit 9 b, irradiation signalacquiring unit 9 c, X-ray image acquiring unit 9 d and lag correctingunit 9 e in Embodiment 1 will be described with reference to the flowchart shown in FIG. 6 and the time chart shown in FIG. 7. Thisprocessing will be described by taking for example what takes place froman end of X-ray irradiation in a preceding imaging event to X-rayirradiation in a current imaging event, and in preparation, for offsetimages and gain correcting images before the imaging events.

(Step S1) Store Offset Images and Gain Correcting Images

The offset images and gain correcting images have different propertiesaccording to storage times for accumulating signal information (electriccharges) corresponding to X-ray detection, signals. Thus, as shown inFIG. 8, these images are stored as corresponding to the respectivestorage times in advance of the imaging events.

In Embodiment 1, and also in Embodiments 2 and 3 described, hereinafter,storage times in time of non-irradiation are regarded as sampling timeintervals (e.g. 1/30 second), represented by cycle ΔT1, for samplingsignals in time of non-irradiation. On the other hand, as noted also inthe section herein “PROBLEMS TO BE SOLVED BY THE INVENTION”, inpractice, the storage time immediately after an image pick-up isdependent on a pulse width of X rays variable in time with the thicknessand the like of patient M. In Embodiment 1, and also in Embodiments 2and 3 described hereinafter. X-ray irradiation time (pulse width of Xrays) for image pickup is synchronized with cycle ΔT1, and is set to twoor more times the cycle ΔT1. In this example, irradiation times (pulsewidths of X rays) enabling image pickup are regarded as three differenttimes, which are ΔT1, ΔT2=ΔT1×2, and ΔT3=ΔT1×3.

An offset image and a gain correcting image are acquired for each ofthese storage times (sampling times and irradiation times). The offsetimages are stored in the offset image memory unit 11 a, and the gaincorrecting images in the gain correcting image memory unit 11 b.

In FIG. 8, ΔT1 corresponds to offset image O1 and gain correcting imageG1 obtained in time of ΔT1, ΔT2 corresponds to offset image O2 and gaincorrecting image G2 obtained in time of ΔT2, and ΔT3 corresponds tooffset image O3 and gain correcting image G3 obtained in time of ΔT3.This step S1 corresponds to the offset image storing step and gaincorrecting image storing step.

(Step S2) Waiting Time Elapsed?

A checking is made whether or not a predetermined waiting time T_(W) haselapsed from the end of X-ray irradiation in the preceding imaging eventas shown in FIG. 7. Immediately after the end of irradiation, alag-behind part includes numerous short time constant components ormedium time constant components. These short or medium time constantcomponents attenuate in a short time. After their attenuation, long timeconstant components become dominant, and remain with substantially thesame intensity. The waiting time T_(W) is provided so that X-raydetection signals may be acquired in time of non-irradiation after lapseof the predetermined time from X-ray irradiation in the precedingimaging event. Upon lapse of the waiting time T_(W), the operationproceeds to next step S3. Whether the waiting time T_(W) has passed ornot may be determined by means of a timer (not shown). That is, thetimer is reset to “0” to start counting simultaneously with thetermination of X-ray irradiation in the preceding imaging event. It maybe determined, when a count corresponding to the waiting time T_(W) isreached, that the waiting time T_(W) has passed.

The waiting time T_(W), preferably, is about 15 seconds although thisdepends on the lag characteristics of individual FPD 3, and the waitingtime T_(W) of about 30 seconds should be sufficient. The longer waitingtime T_(W), e.g. at least 30 seconds, is the better. However, anexcessively long time means an extended interval between imaging events,it is realistic for practical purposes to set the waiting time T_(W) toabout 3 seconds.

(Step S3) Acquire X-Ray Detection Signals in Time of Non-Irradiation

The non-irradiation signal acquiring unit 9 a successively acquiresX-ray detection signals at sampling time intervals ΔT1 in time ofnon-irradiation after lapse of the waiting time T_(W). The number ofsampling times before start of the X-ray irradiation in the currentimaging operation is set to (N+1) (note that K=0, 1, 2, . . . , N−1 andN), with K=0 indicating the first signal acquired immediately afterlapse of the waiting time T_(W). With a (K+1)th X-ray detection, signalregarded as I_(K), the first X-ray detection signal acquired immediatelyafter lapse of the waiting time T_(W) is I₀, and the X-ray detectionsignal acquired immediately before start of X-ray irradiation in thecurrent imaging event is I_(N). It is assumed here that steps S3-S6 aresuccessively executed for each sampling time interval ΔT1.

(Step S4) Current Imaging Reached?

A checking is made whether or not the time for acquiring X-ray detectionsignals in step S3, i.e. sampling time, has reached the start of X-rayirradiation in the current imaging event (whether or not K=N+1). When ithas been reached, the operation jumps to step S7. Otherwise, next stepS5 is executed.

(Step S5) Increment Value of K by 1

The value of subscript K is incremented by 1 for a next sampling.

(Step S6) Discard Preceding X-Ray Detection Signal

X-ray detection signal I_(K) acquired by the non-irradiation signalacquiring unit 9 a in step S3 is written and stored in thenon-irradiation signal memory unit lie. At this time, X-ray detectionsignal I_(K-1) acquired before X-ray detection signal I_(K) is discardedas no longer necessary. Thus, only the latest X-ray detection signalremains stored, in the non-irradiation signal memory unit 11 c. When,the operation proceeds to step S6 after incrementing K=0 to K=1 in stepS5, there exits no X-ray detection signal preceding signal I₀, and thusno signal to be discarded. Then, the operation returns to step S3 for anext sampling, and repeats steps S3-S6 for each of the sampling timeintervals ΔT1. While, in Embodiment 1, preceding X-ray detection signalsare discarded and only the latest X-ray detection signal is retained, itis of course not absolutely necessary to discard the earlier signals.The above steps S3-S6 correspond to the non-irradiation signal acquiringstep in this invention.

(Step S7) Acquire Lag Image

When the sampling time has reached the start of X-ray irradiation in thecurrent imaging event in step S4, the (N+1)th X-ray detection signalI_(N) acquired in step S3 is employed as a lag image. That is, the lagimage acquiring unit 9 b reads from the non-irradiation signal memoryunit 11 c the X-ray detection signal I_(N) acquired immediately beforethe start of X-ray irradiation in the current imaging event, andacquires the X-ray detection signal I_(N) as a lag image. Thus, the lagimage L=I_(N). However, for this lag image L, an offset image and a gaincorrecting image are taken into consideration, and a lag image L′ takingthe offset image and gain correcting image into consideration is derivedfrom the following equation (1):L′=(L−O1)÷G1  (1)

Since the storage time at this time is sampling time ΔT1 in time ofnon-irradiation, the offset image O1 corresponding to sampling time ΔT1is read from the offset image memory unit 11 a, and the gain correctingimage G1 corresponding to sampling time ΔT1 is read from the gaincorrecting memory unit 11 b. The lag image acquiring unit 9 b derives afinal lag image L′ from the above equation (1). The lag image L′acquired by the lag image acquiring unit 9 b is written and stored inthe lag image memory unit 11 d. This step S7 corresponds to the lagimage acquiring step in this invention.

Each of the gain correcting images G1-G3, including the gain correctingimage G1 used, in the above equation (1) and gain correcting image G2used in equation (2) described hereinafter, also includes offsetcomponents. It is therefore preferable to obtain a gain correcting imageG′ by subtracting offset image O from gain correcting image G, asG−O=G′, and to substitute the gain correcting image G′, instead of gaincorrecting image G, into the equations, such as equation (1) andequation (2).

(Step S8) Acquire X-Ray Detection Signals in Time of Irradiation

Upon completion of X-ray irradiation in the current imaging event, theirradiation signal acquiring unit 9 c acquires X-ray detection signalsin time of irradiation resulting from this irradiation. The X-raydetection signals in time of irradiation acquired, by the irradiationsignal acquiring unit 9 c are written and stored in the irradiationsignal memory unit 11 e. This step S8 corresponds to the irradiationsignal acquiring step in this invention.

(Step S9) Acquire X-Ray Image in Current Imaging

The X-ray detection signals in time of irradiation acquired in step S8are referenced X. For the X-ray detection signals X in time ofirradiation, the offset images and gain correcting images are taken intoconsideration, and an X-ray image X′ taking the offset, images and gaincorrecting images into consideration is derived from the followingequation (2).X′=(X−O2)÷G2  (2)

The storage time at this time is set to ΔT2 (=ΔT1×2) which is twice thesampling time (i.e. cycle) ΔT1 in time of non-irradiation as shown inFIG. 7 (see the two-dot chain line in FIG. 7). The offset image O2corresponding to sampling time ΔT2 is read from the offset image memoryunit 11 a, and the gain correcting image G2 corresponding to samplingtime ΔT2 is read from the gain correcting image memory unit 11 b. Then,the X-ray image acquiring unit 9 d derives X-ray image X′ in thisimaging event from the above equation (2). The X-ray image X′ acquiredby the X-ray image acquiring unit 9 d is written, and stored in theX-ray image memory unit 11 f. This step S9 corresponds to the radiationimage acquiring step in this invention. The X-ray image corresponds tothe radiographic image serving an intended purpose in this invention.

(Step S10) Lag Correction

The lag correcting unit 9 e reads the lag image L′ acquired in step S7from the lag image memory unit 11 d, reads the X-ray image X′ acquiredin step S9 from the X-ray image memory unit 11 f, and subtracts the lagimage L′ from the X-ray image X′. An X-ray image Y after the lagcorrection is expressed by Y=X′−L′.

In actual situations, the timing of X-ray irradiation in the currentimaging event is not necessarily determined beforehand. Therefore, thetime of reaching K=N+1 is not necessarily known in advance. Then, inactual situations, steps S3-S6 described above are repeated for eachsampling time interval, and the sampling time reaching the start ofX-ray irradiation in the current imaging event in step S4 is regarded asthe time of reaching K=N+1. Where the timing of X-ray irradiation in thecurrent imaging event is determined in advance, the time of reachingK=N+1 is also known in advance, of course. In such a case, a value of Nmay be set in advance so that the sampling time may reach the start ofX-ray irradiation in the current imaging event in accordance with thetiming of reaching K=N+1. This step S10 corresponds to the lagcorrecting step in this invention.

The technique of removing a lag from X-ray image X′ by using lag imageL′ is not limited to the technique of subtracting lag image L′ directlyfrom X-ray image X′. In Embodiment 1, for example, the storage time ofthe lag image is sampling time ΔT1 in time of non-irradiation, and thestorage time of the X-ray image is ΔT2 (=ΔT1×2) which is twice thesampling time ΔT1. The two images are different in mode regardingstorage time. In such a case, it is preferable to subtract, twice thelag image (however, the gain [amplification factor] of amplifier 38remains unchanged). In this case, the equation becomes Y=X′−2×L′.

According to Embodiment 1 having the described construction, offsetimages are used to carry out offset correction for removing offsetvalues superimposed on signals. The offset images corresponding to aplurality of storage times for accumulating signal information arestored, in the offset, image memory unit 11 a. Gain correcting imagesare used to carry out gain correction for equalizing signal levels ofpixels to be outputted. The gain, correcting images corresponding to theplurality of storage times for accumulating signal information arestored in the gain correcting image memory unit 11 b.

The non-irradiation signal acquiring unit 9 a acquires a plurality ofX-ray detection signals (I₀, I₁, I₂, - - - , I_(N . . . 1), I_(N) inEmbodiment 1) detected from the flat panel X-ray detector (FPD) 3 intime of non-irradiation before irradiation of X rays in an imagingevent. Based on these X-ray detection signals acquired by thenon-Irradiation signal acquiring unit 9 a, the offset, image (O1 in theabove equation (1) in Embodiment 1) stored in the above offset imagememory unit 11 a and corresponding to the storage time (ΔT1 inEmbodiment 1) for the non-irradiation signal acquiring unit 9 a, and thegain correcting image (G1 in the above equation (1) in Embodiment 1)stored in the above gain correcting image memory unit 11 b andcorresponding to the storage time for the non-irradiation signalacquiring unit 9 a, the lag image acquiring unit 9 b acquires a lagimage from the above equation (1).

On the other hand, the irradiation signal acquiring unit 9 c acquiresX-ray detection signals detected from the FPD 3 in time of X-rayirradiation in an imaging event. Based on the X-ray detection signalsacquired by the irradiation signal acquiring unit 9 c, the offset image(O2 in the above equation (2) in Embodiment 1) stored in the aboveoffset, image memory unit 11 a and corresponding to the storage time forthe irradiation signal acquiring unit 9 c, and the gain correcting image(O2 in the above equation (2) in Embodiment 1) stored in the above gaincorrecting image memory unit 11 b and corresponding to the storage time(ΔT2 in Embodiment 1) for the irradiation signal acquiring unit 9 c, theX-ray image acquiring unit 9 d acquires an X-ray image serving anintended purpose from the above equation (2).

Then, the lag correcting unit 9 e carries out a lag correction oflag-behind parts by removing lags, using the lag image acquired by thelag image acquiring unit 9 b, from the X-ray image acquired by the X-rayimage acquiring unit 9 d, thereby removing from the X-ray detectionsignals the lag-behind parts included in the X-ray detection signals.

Thus, there is no need, to carry out lag correction by performingrecursive computations the number of times X-ray detection signals aresampled, as described in Patent Document 2 noted hereinbefore. Further,the lag image forming the basis for the above lag correction, and theX-ray image which, is the target, of the lag correction, take intoconsideration the offset images and lag correcting images correspondingto the respective storage times (ΔT1 and ΔT2 in Embodiment 1). Itbecomes possible to perform appropriately also offset correcting and lagcorrecting images according to the storage times by lag correction. Alag-behind part may therefore be eliminated from an X-ray detectionsignal in a simple way. Further, there is no need to use backlight asused in Patent Document 1 noted hereinbefore. This avoids complicationof the apparatus construction.

In Embodiment 1, and also in Embodiments 2 and 3 to follow, a pluralityof X-ray detection signals are acquired in time of non-irradiation afterlapse of the predetermined time (i.e. the waiting time T_(W) inEmbodiment 1) from X-ray irradiation in a preceding imaging event.Consequently, a plurality of X-ray detection signals are acquired intime of non-irradiation before X-ray irradiation in a current imagingevent. When the X-ray irradiation in the preceding imaging event iscompleted and a transition is made to a state of non-irradiation, shorttime constant components or medium time constant components of alag-behind part attenuate in a short time. After their attenuation, longtime constant components become dominant, and remain with substantiallythe same intensity. Consequently, when an X-ray detection signal isacquired immediately after completion of the X-ray irradiation in thepreceding imaging event, short and medium time constant components areincluded in the signals acquired. The lag-behind part having the shortand medium time constant components cannot be eliminated from the signalaccurately. Thus, in Embodiment 1, a plurality of X-ray detectionsignals are acquired in time of non-irradiation after lapse of thepredetermined time from the X-ray irradiation in the preceding imagingevent. Consequently a plurality of X-ray detection signals are acquiredin time of non-irradiation before X-ray irradiation in the currentimaging event. The signals may be acquired in a state of including onlythe long time constant components which remain after lapse of thepredetermined time. The signals are free from the short and medium timeconstant components, and a lag-behind part having the long time constantcomponents may be eliminated accurately.

Embodiment 2

Next, Embodiment 2 of this invention will be described with reference tothe drawings. Like reference signs will be used to identify like partswhich are the same as in Embodiment 1 and will not be described again. Afluoroscopic apparatus in Embodiment 2 is similar to the apparatus inEmbodiment 1, and only the series of signal processing by thenon-irradiation signal acquiring unit 9 a, lag image acquiring unit 9 b,X-ray image acquiring unit 9 d and lag correcting unit 9 e is differentfrom that in Embodiment 1.

The series of signal processing by the non-irradiation signal acquiringunit 9 a, lag image acquiring unit 9 b, X-ray image acquiring unit 9 dand lag correcting unit 9 e in Embodiment 2 will be described withreference to the flow chart of FIG. 10. Like numerals are affixed tolike steps in Embodiment 1 and will not be described again.

(Step S1) Store Offset Images and Gain Correcting Images

As in Embodiment 1 described hereinbefore, offset images and gaincorrecting images corresponding to storage times (sampling times andirradiation times) are obtained for each storage time. These images arestored in the offset image memory unit 11 a or gain correcting imagememory unit 11 b.

(Step S2) Waiting Time Elapsed?

As in Embodiment 1 described hereinbefore, a checking is made whether ornot the waiting time T_(W) has elapsed from the end of X-ray irradiationin the preceding imaging event. Upon lapse of the waiting time T_(W),the operation proceeds to next step S12.

(Step S12) Acquire X-Ray Detection Signals in Time of Non-Irradiation

As in Embodiment 1 described hereinbefore. X-ray detection signals aresuccessively acquired at sampling time intervals ΔT1 (e.g. 1/30 second)in time of non-irradiation after lapse of the waiting time T_(W). InEmbodiment 2, as will become clear from the following description, thesignals from the first X-ray detection signal I₀ acquired immediatelyafter the waiting time T_(W) to the seventh X-ray detection signal I₆remain stored in the non-irradiation signal memory unit 11 c, instead ofbeing discarded, until acquisition of the eighth X-ray detection signalI₇ (i.e. K=7). It is to be noted that steps S12-S14 are repeated at eachof the sampling time intervals.

(Step S13) K=7?

A checking is made whether or not subscript K has reached 7, that iswhether the sampling time has reached to the eighth (i.e. K=7). WhenK=7, the operation jumps to step S3. Otherwise, next step S14 isexecuted.

(Step S14) Increment Value of K by 1

As in Embodiment 1 described hereinbefore, the value of subscript K isincremented by 1 for a next, sampling. X-ray detection signals I_(K)acquired by the non-irradiation signal acquiring unit 9 a in step S12are successively written and stored in the non-irradiation signal memoryunit 11 c until acquisition of the eighth X-ray detection signal I₇(i.e. K=7). At this time, X-ray detection signal I_(K . . . 1) acquiredbefore X-ray detection signal I_(K) is not discarded but is retained inthe non-irradiation signal memory unit 11 c until eight X-ray detectionsignals accumulate in the non-irradiation signal memory unit 11 a. Then,the operation returns to step S12 for a next sampling, and repeats stepsS12-S14 for each of the sampling time intervals.

(Step S3)-(Step S10)

When the sampling time has reached the start of X-ray irradiation forthe current imaging event in step S13, steps S3-S10 similar toEmbodiment 1 are executed. However, eight X-ray detection signals areconstantly stored in the non-irradiation signal memory unit lie, andwhen the latest X-ray detection signal is newly stored in thenon-irradiation signal memory unit 11 c in step S6, the oldest X-raydetection signal only is discarded. When the sampling time has reachedthe start of X-ray irradiation in the current imaging event in step S4,a lag image L is created based on the eight signals from (N−6)th X-raydetection signal I_(N . . . 7) to (N+1)th X-ray detection signal I_(N)acquired in step S3. Further, a lag image L′ taking the offset image andgain correcting image into consideration is derived from equation (1)above. Specifically, a lag image is derived from an average of thesesignals (L=ΣI_(i)/8, where Σ is a total of i=N−7 to N). The process fromacquisition of the lag image L to the lag correction is the same as inEmbodiment 1, and its description is omitted.

According to Embodiment 2 having the described construction, as inEmbodiment 1, offset images and gain correcting images corresponding toa plurality of storage times (ΔT1, ΔT2 and ΔT3) are stored before animaging event, and a lag image and an X-ray image are acquired based onthese stored images. Then, lag correction is carried out to remove lagsfrom the X-ray image using the lag image. In this way, from the X-rayimage taking info consideration the offset images and gain correctingimages corresponding to the storage times, lags are removed using thelag image which similarly takes into consideration the offset images andgain correcting images corresponding to the storage times. Lag-behindparts, including offset and gain components, are removed from X-raydetection signals in a simple way.

In Embodiment 1, random noise components of X-ray image Y after the lagcorrection become 2^(1/2) times those of image X, thereby lowering thesignal-to-noise ratio by 41% (=(2^(1/2)−1)). In order to suppress thisdeterioration, Embodiment 2, as distinct from Embodiment 1, derives thelag image L by directly using the plurality of X-ray detection signals(I_(N . . . 7), I_(N . . . 6), . . . , I_(N . . . 1) and I_(N) inEmbodiment 2). In this case, the random noise components of X-ray imageY after the lag correction cause deterioration no more than 6% of theX-ray image X before the correction. Thus, the lag correction can beeffected without lowering the signal-to-noise ratio.

In Embodiment 2, the lag image L is obtained by directly using eight.X-ray detection signals. However, the invention is not limited to aparticular number of X-ray detection, signals to be used. Further,although the lag image L is derived from an average of the signals, thelag image L may be derived from a median. A histogram showingintensities of the signals may be formed, to derive a mode as lag imageL from the histogram. Thus, the invention is not limited to a particularway of deriving the lag image L.

Embodiment 3

Next, Embodiment 3 of this invention will be described with reference tothe drawings. FIG. 11 is a schematic view showing flows of data to andfrom an image processor and a memory in Embodiment 3. Like referencesigns will be used to identify like parts which are the same as inEmbodiments 1 and 2, and will not be described again. A fluoroscopicapparatus in Embodiment 3 is the same as the apparatus in Embodiments 1and 2, except the flows of data to and from the image processor 9 andmemory 11 shown in FIG. 11. The series of signal processing by thenon-irradiation signal acquiring unit 9 a, lag image acquiring unit 9 b,irradiation signal acquiring unit 9 c, X-ray image acquiring unit 9 dand lag correcting unit 9 e also is different from those in Embodiments1 and 2.

In Embodiment 3, as shown in FIG. 11, the lag image acquiring unit 9 bacquires a lag image L (before taking the offset images and gaincorrecting images into consideration) by recursive computation based onthe X-ray detection signals in time of non-irradiation read from thenon-irradiation signal memory unit 11 c and a preceding lag image readfrom the lag image memory unit 11 d. The acquisition of a lag image L byrecursive computation will be described with reference to the flow chartof FIG. 12. The lag image acquiring unit 9 b acquires a lag image L′using equation (1) above, based on the offset images and gain correctingimages, through recursive computation. The lag correcting unit 9 eremoves the lag image read from the lag image memory unit 11 d from anX-ray image obtained in the current imaging event, in the same way as inEmbodiments 1 and 2 described hereinbefore.

Next, the series of signal processing by the non-irradiation signalacquiring unit 9 a, lag image acquiring unit 9 b, irradiation signalacquiring unit 9 c, X-ray image acquiring unit 9 d and lag correctingunit 9 e in Embodiment 3 will be described with reference to the flowchart of FIG. 12. Like numerals are affixed to like steps in Embodiments1 and 2 and will not be described again.

(Step S1) Store Offset Images and Gain Correcting Images

As in Embodiments 1 and 2 described hereinbefore, offset images and gaincorrecting images corresponding to storage times (sampling times andirradiation times) are obtained for each storage time. These images arestored in the offset image memory unit 11 a or gain correcting imagememory unit 11 b.

(Step S2) Waiting Time Elapsed?

As in Embodiments 1 and 2 described hereinbefore, a checking is madewhether or not the waiting time T_(W) has elapsed from the end of X-rayirradiation in the preceding imaging event. Upon, lapse of the waitingtime T_(W), the operation proceeds to next, step S22.

(Step S22) Acquire X-Ray Detection Signal Immediately after Waiting Time

As in Embodiments 1 and 2 described hereinbefore. X-ray detectionsignals are successively acquired at sampling time intervals ΔT1 (e.g.1/30 second) in time of non-irradiation after lapse of the waiting timeT_(W). The first X-ray detection signal I₀ is acquired immediately afterthe waiting time T_(W), which is written and stored in thenon-irradiation signal memory unit 11 c.

(Step S23) Acquire Lag Image of Initial Value

The lag image acquiring unit 9 b reads this X-ray detection signal I₀from the non-irradiation signal memory unit 11 c, and acquires from theX-ray detection signal I₀ a lag image L₀ as an initial value of lagimage L (before taking the offset images and gain correcting images intoconsideration). The lag image L₀ of initial value acquired by the lagimage acquiring unit 9 b is written and stored in the lag image memoryunit 11 d.

(Step S3)-(Step S10)

After the lag image L₀ of initial value is acquired in step S23, stepsS3-S10 similar to Embodiment 1 are executed. However, the X-raydetection signals acquired in time of non-irradiation in step S3 are thesecond X-ray detection signal I₁ et seq. When acquiring the lag image L(before taking the offset images and gain correcting images intoconsideration) in step S7, an (N+1)th lag image L_(N) is derived byrecursive computation from the X-ray detection signals I_(N) in time ofnon-irradiation read from the non-irradiation signal memory unit 11 cand the preceding lag image L_(N . . . 1) read from the lag image memoryunit 11 d. In Embodiment 3, the lag image L_(N) is derived by arecursive weighted average (hereinafter referred to as “recursiveprocess” as appropriate) from the following equation (3):L _(N)=(1−P)×L _(N . . . 1) +P×I _(N)  (3)

In this process, I₀=L₀ as noted above. P is a load ratio which takes avalue of 0 to 1.

When the latest lag image L_(N) is acquired as lag image L in step S7,only the lag image L_(N . . . 1) preceding the lag image L_(N), i.e.only the lag image L_(N . . . 1) serving as the basis of the recursiveprocess expressed by equation (3) above, is required. The remaining lagimages L, i.e. lag image L_(N . . . 2) before last and lag imagesL_(N . . . 3), . . . , L₁ and L₀ acquired earlier are unnecessary. Thus,once the latest lag image L_(N) is stored in the lag image memory unit11 d, only the immediately preceding lag image L_(N . . . 1) is retainedand the other lag images L are discarded. It is of course not absolutelynecessary to discard the lag image L_(N . . . 2) before last and earlierlag image L_(N . . . 3) and so on.

Further, the latest lag image L_(N) obtained through the series ofrecursive processes is regarded as L, and based on the offset images andgain correcting images, the lag image acquiring unit 9 b acquires, usingequation (1) above, lag image L′ taking the offset images and gaincorrecting images into consideration.

According to Embodiment 3 having the described construction, as inEmbodiments 1 and 2, from the X-ray image taking into consideration theoffset images and gain correcting images corresponding to the storagetimes, lags are removed using the lag image which similarly takes intoconsideration the offset images and gain correcting images correspondingto the storage times. Lag-behind parts, including offset and gaincomponents, are removed from X-ray detection signals in a simple way.

In Embodiment 3, a plurality of X-ray detection signals are successivelyacquired at sampling time intervals ΔT1 (e.g. 1/30 second) in time ofnon-irradiation. Assuming a certain point in time of non-irradiation tobe the (N+1)th, a lag image L is obtained based on a plurality of X-raydetection signals including the (N+1)th signal so far acquiredsuccessively. That is, an (N+1)th lag image L_(N) is obtained. For thispurpose, the recursive computation is repeated based on the X-raydetection signal I_(N) acquired at the (N+1)th point of time, and a lagimage L (before taking the offset images and gain correcting images intoconsideration) based on a plurality of X-ray detection signalssuccessively acquired up to the Nth point, of time before the (N+1)thpoint of time, that is the lag image L_(N . . . 1) before the lag imageL_(N).

Whenever an X-ray detection signal is successively acquired in time ofnon-irradiation, the recursive computation is repeated based on thelatest X-ray detection signal I_(N) acquired, and the lag image (i.e.preceding lag image) L_(N . . . 1) resulting from a plurality of X-raydetection signals successively acquired in the past. The lag image L_(N)ultimately obtained is a lag image L before taking the offset images andgain correcting images into consideration. Further, the lag image L′derived from equation (1) above and taking the offset images and gaincorrecting images into consideration is a goal image used as the basisfor the lag correction. Only the newest lag image L_(N) obtained byrecursive computation and the lag image (i.e. the lag image used as thebasis of the recursive computation) L_(N . . . 1) before the lag imagemay be retained, with the other lag images (i.e. the lag images earlierthan the above two lag images) discarded. Then, for example, the lagimage memory unit 11 d may have a storage region just for two frames,i.e. enough for storing two images. This provides an advantage ofsimplifying the construction.

In Embodiment 3, the lag image is obtained by recursive process (seeequation (3) above) which is a recursive weighted average as recursivecomputation, which realizes a lag correction with increased reliability.Regarding the S/N ratio, as shown in FIG. 13, when the load ratio P inequation (3) above is 0.25 (see the solid line in FIG. 13) the randomnoise components are reduced to 0.39 by repeating the recursivecomputation eight times or more. The random noise components of X-rayimage Y after the lag correction cause a deterioration not exceeding 7%which is almost the same as the 6% in Embodiment 2 describedhereinbefore where the lag image is obtained by directly using eightX-ray detection signals. Thus, the lag correction can be effectedwithout lowering the S/N ratio.

This invention is not limited to the foregoing embodiments, but may bemodified as follows:

(1) In each embodiment described above, a fluoroscopic apparatus asshown in FIG. 1 has been described by way of example. This invention maybe applied also to a fluoroscopic apparatus mounted on a C-shaped arm,for example. This invention may be applied also to an X-ray CTapparatus. This invention is useful particularly when actual photography(rather than fluoroscopy) is carried out as by an X-ray radiographicapparatus.

(2) In each embodiment described above, the flat panel X-ray detector(FPD) 3 has been described by way of example. This invention isapplicable to any X-ray detectors in wide use.

(3) In each embodiment described above, the X-ray detector for detectingX rays has been described by way of example. This invention is notlimited to a particular type of radiation detector which may, forexample, be a gamma-ray detector for detecting gamma rays emitted from apatient dosed with radioisotope (RI), such as in an ECT (EmissionComputed Tomography) apparatus. Similarly, this invention is applicableto any imaging apparatus that detects radiation, as exemplified by theECT apparatus noted, above.

(4) In each embodiment described above, the FPD 3 is a direct conversiontype detector with a radiation (X rays in the embodiments) sensitivesemiconductor for converting incident radiation directly into chargesignals. Instead of the radiation sensitive type, the detector may bethe indirect conversion type with a light sensitive semiconductor and ascintillator, in which incident radiation is converted. Into light bythe scintillator, and the light is converted into charge signals by thelight sensitive semiconductor.

(5) In each embodiment described above, an operation is started toacquire X-ray detection signals in time of non-irradiation after lapseof the predetermined time (i.e. the waiting time T_(W) in eachembodiment) from X-ray irradiation in a preceding imaging event. Wherethe short and medium time constant, components are at a negligiblelevel, the acquisition of X-ray detection signals may be startedsimultaneously with a transition from the X-ray irradiation in thepreceding imaging event to the non-irradiation state. This applies alsoto radiation other than X rays.

(6) In each embodiment described above, the lag image serving as thebasis for the lag correction includes data of X-ray detection signalI_(N) acquired immediately before a start of X-ray irradiation in thecurrent imaging event. It is not absolutely necessary to include thedata of X-ray detection signal I_(N). However, since the latest data isthe most reliable, it is desirable, as in each embodiment, to obtain alag image including the data of X-ray detection signal I_(N), andperform the lag correction by removing lags using the lag image. Thisapplies also to radiation other than X rays.

(7) In each embodiment described above, based on both the offset imageand gain correcting image, a lag image and an X-ray image are acquiredtaking both images into consideration. Instead, based only on one of theoffset image and gain correcting image, a lag image and an X-ray imagemay be acquired taking only one of the images into consideration. Thisapplies also to radiation other than X rays.

(8) In each embodiment described above, in order to take the offsetimage and gain correcting image into consideration, equation (1) aboveis used for deriving the lag image, and equation (2) above is used forderiving the X-ray image. A technique of taking the offset image andgain correcting image into consideration in a usual method is notlimited to the subtraction or division as in equation (1) or (2) above.

(9) Embodiment 3 described above employs the recursive weighted, average(recursive process) as shown in the foregoing equation (3). Therecursive computation is not limited to the recursive weighted average,but may be an inveighed recursive computation. Thus, function f (I_(N),L_(N−1)) expressed by X-ray detection signal I_(N) and lag image L_(N−1)may be expressed by the lag image L_(N) to serve the purpose.

INDUSTRIAL UTILITY

As described above, this invention is suited to a radiographic apparatushaving a flat panel. X-ray detector (FPD).

1. A radiographic apparatus for obtaining radiographic images based onradiation detection signals, comprising: a radiation emitting device foremitting radiation toward an object under examination; a radiationdetecting device for detecting radiation transmitted through the object;an offset image storage device for storing offset images correspondingto a plurality of storage times for accumulating information on signals,the offset images being used to perform offset correction for removingoffset values superimposed on the signals; a non-irradiation signalacquiring device for acquiring a plurality of radiation detectionsignals detected from the radiation detecting device in time ofnon-irradiation before irradiation of the radiation in an imaging event;a lag image acquiring device for acquiring a lag image based on theplurality of radiation detection signals acquired by the non-irradiationsignal acquiring device, and the offset images stored in said offsetimage storage device and corresponding to storage times for thenon-irradiation signal acquiring device; an irradiation signal acquiringdevice for acquiring radiation detection signals detected from theradiation detecting device in time of irradiation of the radiation inthe imaging event; a radiographic image acquiring device for acquiring aradiographic image based on the radiation detection signals acquired bythe irradiation signal acquiring device, and the offset images stored insaid offset image storage device and corresponding to the storage timesfor the irradiation signal acquiring device; and a lag correcting devicefor removing lags, using the lag image acquired by said lag imageacquiring device, from the radiographic image acquired by theradiographic image acquiring device, thereby performing a lag correctionof lag-behind parts by removing the lag-behind parts from the radiationdetection signals.
 2. A radiographic apparatus for obtainingradiographic images based on radiation detection signals, comprising: aradiation emitting device for emitting radiation toward an object underexamination; a radiation detecting device for detecting radiationtransmitted through the object; a gain correcting image storage devicefor storing gain correcting images corresponding to a plurality ofstorage times for accumulating information on signals, the gaincorrecting images being used to perform gain correction for equalizingsignal levels of pixels to be outputted; a non-irradiation signalacquiring device for acquiring a plurality of radiation detectionsignals detected from the radiation detecting device in time ofnon-irradiation before irradiation of the radiation in an imaging event;a lag image acquiring device for acquiring a lag image based on theplurality of radiation detection signals acquired by the non-irradiationsignal acquiring device, and the gain correcting images stored in saidgain correcting image storage device and corresponding to storage timesfor the non-irradiation signal acquiring device; an irradiation signalacquiring device for acquiring radiation detection signals detected fromthe radiation detecting device in time of irradiation of the radiationin the imaging event; a radiographic image acquiring device foracquiring a radiographic image based on the radiation detection signalsacquired by the irradiation signal acquiring device, and the gaincorrecting images stored in said gain correcting image storage deviceand corresponding to the storage times for the irradiation signalacquiring device; and a lag correcting device for removing lags, usingthe lag image acquired by said lag image acquiring device, from theradiographic image acquired by the radiographic image acquiring device,thereby performing a lag correction of lag-behind parts by removing thelag-behind parts from the radiation detection signals.
 3. A radiationdetection signal processing method for performing a signal processing toobtain radiographic images based on radiation detection signals detectedby irradiating an object under examination, said signal processingcomprising: an offset image storing step for storing, before an imagingevent, offset images corresponding to a plurality of storage times foraccumulating information on signals, the offset images being used toperform offset correction for removing offset values superimposed on thesignals; a non-irradiation signal acquiring step for acquiring aplurality of radiation detection signals in time of non-irradiationbefore irradiation of the radiation in the imaging event; a lag imageacquiring step for acquiring a lag image based on the plurality ofradiation detection signals acquired in the non-irradiation signalacquiring step, and the offset images stored in said offset imagestorage step and corresponding to storage times in the non-irradiationsignal acquiring step; an irradiation signal acquiring step foracquiring radiation detection signals in time of irradiation of theradiation in the imaging event; a radiographic image acquiring step foracquiring a radiographic image based on the radiation detection signalsacquired in the irradiation signal acquiring step, and the offset imagesstored in said offset image storage step and corresponding to thestorage times in the irradiation signal acquiring step; and a lagcorrecting step for removing lags, using the lag image acquired in saidlag image acquiring step, from the radiographic image acquired in theradiographic image acquiring step, thereby performing a lag correctionof lag-behind parts by removing the lag-behind parts from the radiationdetection signals.
 4. A radiation detection signal processing method forperforming a signal processing to obtain radiographic images based onradiation detection signals detected by irradiating an object underexamination, said signal processing comprising: a gain correcting imagestoring step for storing, before an imaging event, gain correctingimages corresponding to a plurality of storage times for accumulatinginformation on signals, the gain correcting images being used to performgain correction for equalizing signal levels of pixels to be outputted;a non-irradiation signal acquiring step for acquiring a plurality ofradiation detection signals in time of non-irradiation beforeirradiation of the radiation in the imaging event; a lag image acquiringstep for acquiring a lag image based on the plurality of radiationdetection signals acquired in the non-irradiation signal acquiring step,and the gain correcting images stored in said gain correcting imagestorage step and corresponding to storage times in the non-irradiationsignal acquiring step; an irradiation signal acquiring step foracquiring radiation detection signals in time of irradiation of theradiation in the imaging event; a radiographic image acquiring step foracquiring a radiographic image based on the radiation detection signalsacquired in the irradiation signal acquiring step, and the gaincorrecting images stored in said gain correcting image storage step andcorresponding to the storage times in the irradiation signal acquiringstep; and a lag correcting step for removing lags, using the lag imageacquired in said lag image acquiring step, from the radiographic imageacquired in the radiographic image acquiring step, thereby performing alag correction of lag-behind parts by removing the lag-behind parts fromthe radiation detection signals.